At least three people were killed and dozens more wounded when bombs exploded in two buses on a busy highway in the Kenyan capital Nairobi.

Two privates buses were ripped apart by what is believed to be improvised explosive devices.

More than 60 people were taken to hospital, some of them in critical condition.

It is the second attack in Kenya in as many days after eight people died in three explosions in the coastal city of Mombasa on Saturday.

"Security agencies are in pursuit of the perpetrators of this heinous and cowardly act," Kenya's vice-president William Ruto said about the latest bombings.

Kenyan media reports said bombs appeared to have been planted on the buses, while unconfirmed reports said powerful grenades may have been thrown at them from the side of the road.

The buses blew up along the Thika Road highway, an area around eight kilometres north-east of Nairobi's city centre.

An AFP reporter at the scene saw a red passenger bus with a large hole in its side, and with the ripped panels spattered in blood.

Kenyan media also showed images of a green bus with its roof and sides buckled by an explosion.

The Nation newspaper said the 45-seater buses were almost full when the blasts occurred.

Somali terrorist group Al Shabaab intensifies campaign

The east African country has become increasingly dangerous in recent months.

The Somali terrorist group Al Shabaab has intensified its campaign - hoping to get Kenyan troops to withdraw from peacekeeping in Somalia.

The Islamist group claimed responsibility for the high-profile attack on Nairobi's Westgate shopping mall last year in which at least 67 people were killed.

In March, two people were arrested in Mombasa along with a car expertly packed with explosives.

Intelligence sources say they believe the car was rigged in Somalia and driven into Kenya for a high-profile bombing.

Nairobi and Mombasa have also seen a string of smaller bombings and shootings blamed on Islamists, pushing national security to the top of the agenda in the east African nation - which once proudly identified itself as a bastion of stability in the region.

Kenyan police have also been under fire for an ongoing crackdown in Nairobi which has seen thousands of people detained - most of them Somalis or ethnic Somalis.

The operation has focused on Nairobi's main Somali district Eastleigh, and residents have accused police of indiscriminately arresting people of Somali origin.